Patent GB-1492747-A (GB)
Title: Electronic screening
IPC: H04N1/40
Filed: 1974-11-06
Abstract: 1492747 Copying by scanning PRINTING DEVELOPMENTS Inc 6 Nov 1974 [12 Nov 1973] 47914/74 Heading H4F In a system for producing a half tone image in the form of a generally regular array of dots of a size determined by the photo-electric scanning of an original, indications of the presence and type of tone density gradients and edges in the original are obtained by comparing the outputs of additional photo-electric devices directed respectively to an area including the instantaneously scanned point, and to at least one adjacent area, and in response thereto the recorded dots are moved from their nominal positions towards the denser part of the image so that they are better aligned at tone density transitions. As described, the original and recording medium are mounted on drums for synchronous helical scanning. A line of light parallel to the drum axis is transmitted or reflected by the original to a beam splitter. One output of the splitter is sharp-masked in the fast scan direction and unsharp-masked in the slow direction by a rectangular aperture (aspect ratio ~ 1: 20) and transmitted by beam splitting or dichroic mirrors to yellow, magenta and cyan photomultipliers for producing Y, M, C and black printing signals via a computer. The other output of the beam splitter falls on a centre photodiode C, directed to the scanned point, and leading and trailing photodiodes L and T symmetrically spaced either side of photodiode C in the slow scan direction. Diodes L and T are selected from four like pairs with different spacings from the diode C (i.e. nine diodes in all, in a row of a length preferably comparable to the width of a half tone dot), according to which of four predetermined screen rulings is selected. As indicated in Fig. 21, signals X 1 -X 4  are developed according as L > C; C > T; C > L; T > C; and in response thereto control signals with values of + 3 (X 2  only); + 2(X 1 , X 2 ); + 1(X 1 ); - 1(X 4 ); - 2(X 3 , X 4 ); - 3(X 3 ); may be formed. If C is greater than, equal to, or less than, both L and T no signal is formed. Positive voltages indicate a dark to light transition in the slow scan direction. The control signals are multiplied by the black printing signal to give a shift signal controlling the relative shift of the recorded dot from its nominal position, Fig. 6B (not shown). Dot formation.-Although engraving is mentioned, in the preferred embodiment the dots are recorded photographically using a laser beam of elongate cross section, the length and lengthwise position of which are respectively controlled by two galvanometer mirrors with an intermediate knife edge or mask, Figs. 8 and 9 (not shown). The screen angle for each colour is different and is selected from six preset alternatives. Up to 50% intensity the dots are square with edges at 45 degrees to the screen angle and are thereafter generally polygonal, Figs. 13, 15 and 16 (not shown). Dot forming circuits, Figs 3-5 (not shown), for each screen angle receive the colour printing signal and when enabled produce ramp potentials e 13  and e 15 , Fig. 12 (not shown), respectively controlling the length and the position of one end of the laser beam. The potential e 13  is applied directly to the dot width controlling galvanometer and when the dot forming circuitry is not enabled, rides on a bias pedestal ensuring complete blocking of the laser beam. So that dot formation is unaffected by recording speed, some reference voltages used in forming the ramps are produced by conversion of the frequency of a sync signal from the recording drum. In general, a dot centre of the regular half tone screen will not coincide with a helical scan line, and its commencement is a function of the printing signal. A computer determines the precise co-ordinates of the dot centres, and enables a dot forming circuit when the fast-scan co-ordinate of a respective adjacent dot is reached, as determined by the sync signals from the recording drum. It also produces a signal representative of the slow scan deflection necessary to reach the nominal dot position. To this latter signal are added the signal e 15  from the dot forming circuit and the shift signal as determined by the occurrence of tone density edges, the sum being applied to the dot position controlling galvanometer. In alternative embodiments, the intended regular dot positions are prerecorded on magnetic tape or obtained by scanning an existing half tone screen; two, or more than three photo-diodes may be used for developing dot offset signals; the galvanometers may be replaced by acousto-optic modulators.